Skies of Arcadia
Sega is sitting on one of the most well-received RPGs of all-time and has refused to do anything with it since the port of the lone game in the series to the GameCube.
There are so many talented RPG developers both within and outside of Sega, but in this case, Sega happens to have everything they need right in their sphere of influence; and that makes Skies of Arcadia’s neglect very perplexing.
Members of the Skies of Arcadia team are still scattered about Sega, including the legendary Rieko Kodama, and others like Kenji Hiruta have expressed interest in returning to work on Skies of Arcadia.
The teams behind Sakura Wars and Valkyria Chronicles could work together to work on a potential new game under the guidance Kodama and other former developers, and lets not forget that Atlus is part of the Sega family now and could potentially be tapped in to assist as well.
And finally, if Sega put out feelers to external development teams and let people know that they were interested in doing a new Skies of Arcadia game, I would bet they would find plenty of interested parties willing to help.